0:00:06.800,0:00:09.060
As the sun's rays reach [br]the Earth's surface

0:00:09.320,0:00:11.590
some are absorbed and re-emitted as heat

0:00:11.910,0:00:15.080
greenhouse gases such as water [br]vapor and carbon dioxide

0:00:15.300,0:00:17.830
absorb and re-radiate some of this heat

0:00:18.420,0:00:21.660
increased amounts of greenhouse gases [br]in the atmosphere mean more

0:00:21.660,0:00:22.500
heat is trapped

0:00:22.950,0:00:23.780
warming the Earth

0:00:24.510,0:00:25.330
human activities

0:00:25.600,0:00:28.490
especially the burning of fossil [br]fuels have increased

0:00:28.600,0:00:32.160
concentrations of atmospheric [br]carbon dioxide by 40%

0:00:32.560,0:00:33.940
mainly since 1900

0:00:34.540,0:00:38.840
global average surface temperature [br]has increased by 0.8° C over that time

0:00:40.300,0:00:42.770
Other changes to the climate [br]in recent decades,

0:00:42.770,0:00:45.050
can be seen in the warming of the oceans.

0:00:45.050,0:00:46.230
A rise in sea level

0:00:47.010,0:00:49.730
decreasing snow and ice cover [br]in the northern hemisphere

0:00:50.400,0:00:52.520
and a decline in sea ice in the Arctic.

0:00:54.620,0:00:58.480
If emissions continue unchecked [br]then further warming of 2.6 to 4.8° C

0:00:59.610,0:01:02.880
would be expected by [br]the end of the century.

0:01:02.880,0:01:04.090
Even at the low end.

0:01:04.090,0:01:07.000
This would have serious implications [br]for human societies

0:01:07.240,0:01:08.240
and the natural world.

0:01:12.470,0:01:15.140
For more information about climate [br]change from leading science academies,

0:01:16.030,0:01:16.670
please visit

0:01:16.920,0:01:19.160
royalsociety.org/climatechange

0:01:20.160,0:01:23.070
or nas-sites.org/americasclimatechoices